Not able to install with apt-get

I tried to run sudo apt-get install sagemath. It gives below error under ubuntu 16.04:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package sagemath is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

this may work to get sage running in a cheap way... (I hope the ppa still exists. It is very old, the warning was already mentioned. But this fits well with the preference of an old operating system. I also have the bedroom laptop in this configuration. For the mobile daily working machine long standing problems of installing sage on ubuntu > 16.04 forced me to move to manjaro. Now, sagemath ubuntu 18 will be my next try...)